The present invention is directed towards a cylinder former having a variable hydraulic pulse whilst drainage, for use in papermaking.
Today there are numerous ways of forming continuously a sheet of paper or paperboard, for example the use of a number of separate forming sections. The capital cost required to install one of the multifoudrinier is high and sometimes the change is not feasible because of the total capital required. Accordingly, in certain applications, the use of a cylinder mould in formation is desirable.
The principle of sheet formation on a cylinder mould is as follows. A horizontal cylinder (cylinder mould) having a wire cloth surface is arranged to rotate approximately three quarters submerged in a container (vat) of paper stock so that a small arc of its circumference is above stock level. Water associated with the fibrous suspension drains through the wire cloth with the result that a layer of fibers is deposited on the surface. Drainage take place because of a difference in level between the stock in the vat and the back water inside the mould.
A moving felt (mould felt/making felt) is then pressed by means of a roll (couch roll) into a contact with the cylinder at approximately the top position. By doing this the layer of fibers that has formed on the wire screen is transferred to the mould felt which moves away from the forming screen with it. Once the web has been transferred, the wire of the cylinder mould is washed by sprays and re-enters into the fiber stock where a new web is going to be formed.
If a number of these units are placed in series, then a multi-ply web or sheet of paper is produced continuously. Each forming unit typically has its own supply of paper stock and a method of removing the drainage water from its interior so that, in effect, each cylinder mould is a separated web forming machine in itself.
Various types of cylinder mould or vat arrangements currently exist. In this regard, a typical cylinder mould is constructed around a cast iron core upon that are secured bronze supporting spokes known as spiders. The spiders support concentric rims, the outside peripherals of which are grooved in order to carry rods that are approximately 1 centimeter in diameter and approximately 3.5 centimeters apart parallel with the axis of the central shaft. A continuous wire is wound round the cylinder.
On this skeleton is commonly sewn a bronze or stainless steel backing wire. It is over this backing wire that the forming wire is stretched and secured.
Another type of arrangement is what is known as contraflow vat where the stock flows opposite to that of the rotation of the mould. In this regard, the stock from the flow distribution arrangement enters the side at the bottom of the vat, passes over a weir and then over a baffle, rising again to be fed into the vat circle via wing boards (butterfly) and a making board. The purpose of the wing board is to help to correct the basis weight levels, when they have the tendency to be lighter or heavier on one side or the other.
In a uniflow vat, the basic components are essentially the same as for a contraflow vat, but the stock flows with the direction of the mould rotation.
In a dry vat situation, the dry vat has a seal introduced into the vat circle so that the fiber suspension is confined to a shorted section of a vat circle. Because the forming length has been reduced in size, the degree of uncontrolled turbulence is decreased.
In a restricted flow vat or half vat, it is essentially a dry vat with the unused half removed.
In the case of the contraflow vat, the stock enters the vat at considerable turbulence but in a short time becomes less turbulent and moves slowly through the vat towards the opposite side. This is the point where the forming surface of the mould enters the stock and where the major portion of the web formation is taking place. It is found that, in this zone, suspension is practically stationary and the stock is in an extremely flocculated state. Adjacent to the rotating mould surface a boundary layer is formed which moves rapidly in the direction of the cylinder rotation. The thickness of this layer depends on the consistency of the stock, its freeness and machine speed. Continued drainage without a corresponding fiber deposition leads to the consistency in this layer increasing to become substantially higher than that of the inlet stock. This stream of high consistency stock follows the cylinder surface to the point where the mould surface emerges. Here it mixes with the incoming stock and is recirculated to the other side of the cylinder thus increasing the consistency.
Between the two streams of stock mentioned above, an unstable layer is formed and localized differences in velocities are created which lead to a continuous exchange of stock between the two streams. This in turn leads to a non-uniform flow velocity and a non-uniform consistency across the machine that gives uneven conditions influencing both the web formation and the stock wash-off at the line of emergence.
In the case of the uniflow vat, at its inlet there is a turbulent flow that extends over the entire vat section, but this turbulence diminishes as the stock flows downward towards the center of the vat. It is during this first phase at the inlet that the rapid preliminary formation takes place. Some time later, when the flow velocity through the wire has decreased to a certain level, a boundary layer is formed that travels with and approximately at the velocity of the mould surface. This layer transports to the side of the vat a sufficiently large volume of stock to cause stagnation of other layers close to the walls of the vat. As in the case of the previously mentioned contraflow vat, wash-off takes place and results in the elevated consistency of the boundary layer. Where the mould surface leaves the stock, some thickened stock separates from the cylinder, some of this being discharged at the overflow while the remainder flows back downwards into the vat. The consistency of this stock is higher than that at the boundary layer. Counterflow and boundary layer are separated by an unstable intermediate layer through which thickened stock from the counterflow stream is fed back irregularly onto the boundary layer stream. This has a negative effect on web formation. The level differences between the vat and the inside cylinder level, the freeness, the machine speed and the amount of overflow control the intensity of the counterflow.
A rotoformer or sandy hill former consists of an open-ended perforated suction cylinder that is covered by a coarse backing wire and a fine face wire. Inside the cylinder are adjustable compartmented boxes into which drainage takes place under controlled conditions. There is also an initial draining zone at the beginning of web formation where draining is by means of gravity. The pond regulator can have its position adjusted in order to change the stock velocity and pressure applied at the initial forming zone.
The forming length is very short, 10 to 25 centimeters, while the drainage flow rate in the forming zone is very high limiting the basis weight and consistency that this former can handle.
A cylinder suction former consists essentially of a tapered stock inlet system from which tubes feed the stock to a dispersion chamber, followed by a top lid which can be adjusted on the run. Web formation takes place between the top lid and surface of the mould. The position of the suction box can be adjusted on the run. The forming length is very short, 10 to 25 centimeters, while the drainage flow rate in the forming zone is very high limiting the basis weight and consistency that this former can handle.
A short pressure former is a combination of a well-designed stock inlet with an explosion chamber feeding directly into a forming zone. The fiber suspension passes from a tapered inlet through a series of shear pipes into a small compartment, known as the explosion chamber, where the fiber dispersion takes place. Finally, the dispersed fibrous suspension passes to the forming zone where it is confined between a hinged lid and the mould surface. In this case, formation takes place under pressure. The forming length is very short, 10 to 25 centimeters. The drainage flow rate in the forming zone is very high limiting the basis weight and consistency that this former can handle.
Examples of some of the foregoing with modifications can be found in the following patents:
U.S. Pat. Nos:
1,801,238
1,870,971
3,021,899
3,091,563
3,111,454
3,272,692
4,543,159
While the types of cylinder mould arrangements as aforenoted have particular advantages, they also have attendant disadvantages some of which have certain been mentioned.
It is therefor a principal object of the invention to overcome the shortcomings of the devices heretofore mentioned.
It is a further object of the invention to provide for a hydraulic pulsing of the stock that is going to form the sheet to enhance stock distribution whilst also providing for drainage.
The formation of the sheet is the result of physical interaction during the forming process. There are three important hydrodynamic processes during the sheet formation. These processes are drainage, shear and turbulence.
The drainage process has two stages, one is filtration and other is thickening. Filtration is obtained when in the early part of the forming zone a high rate of water removal is achieved, the fines retention is high but shear is not present during this process. Thickening is obtained when small amounts of water are removed. During this process, fines retention is low.
The shear process is the result of controlling the differential speed between the stock flow and the forming (mould) machine. This process has to be controlled accurately or the final sheet will not have the desired properties.
The turbulence is present when the fibers in the stock flow are well dispersed at any consistency and the two hydrodynamic processes above described are present at the same time.
An additional objective of the invention is to provide the combination of the three aforementioned hydrodynamic processes in one forming zone and all of them interacting at the same time, the design of the forming zone will vary depending upon the particular operation.
In this regard, the present invention provides for a cylinder mould former which utilizes an adjustable contoured section in its forming zone. The contoured section provides for control over the ratio between the fiber suspension velocity and the cylinder mould velocity. The MD/CD ratio of the paper improves and becomes lower which is similar to that of a fourdrinier paper machine. At any given position of the contoured section, the fiber suspension flow is subject to continuous hydraulic pulses so the water is forced to pass in and out of the mould. The contoured section is graduated so as to eliminate flow separation due to shear at the boundary layers. The sheet formation occurs as a result of the gently pulsation of the stock slurry and the gradual removal of water as the water/fiber mixture moves towards the discharge lip near the top of the cylinder. This process will decrease or eliminate the filtration process, thus uniformly distributing fines across the thickness of the newly formed sheet. In addition, a baffle arrangement is provided to avoid stock build-up at the bottom of the former.